Rounding to Specific Number of Significant Figures

Date: 05/30/2003 at 21:59:20
From: Amber
Subject: How to round off to specific significant figures
I am not sure how to round off properly if the question is, for
example, asking me to round off to five significant figures... what
do I do?
I am confused as to what digits are considered significant figures.

Date: 05/30/2003 at 23:37:14
From: Doctor Peterson
Subject: Re: How to round off to specific significant figures
Hi, Amber.
See the Dr. Math archives:
Significant Figures, Significant Digits
http://mathforum.org/library/drmath/sets/select/dm_sig_digits.html
Significant Digits and Zero
http://mathforum.org/library/drmath/view/57160.html
The basic idea is that all digits starting with the first (leftmost)
non-zero digit are significant (that is, their value counts). In the
case of numbers like 200, with zeros between the last non-zero digit
and the (implied) decimal point, it is not clear from the way the
number is written whether they are significant; you would have to be
told. That is why significant digits should really be counted only
when a number is written in scientific notation, where there can be
only one digit to the left of the decimal point anyway.
Let's take an example: I'll round 102.0304 to four significant digits.
We count starting at the first non-zero digit: 1,0,2,0,3,0,4. So our
example has seven significant digits.
The only digits in a number like this (with a decimal point in the
middle) that could be insignificant would be zeros at the beginning,
like this: 00102.0304. The first two zeros don't contribute anything
to the value of the number; it would still have seven significant
digits. Zeros at the end, like 102.030400, would be significant,
because they tell us that whoever made the measurement did read zeros
in those digits (if he isn't lying).
We want to round to four; so we chop off the last three (which are
the LEAST significant digits, since they add only a fraction to the
value of the number). Now we have
102.0304
xxx
102.0
This has four significant digits. (Even the last zero is significant,
because it tells us that the number of tenths is zero.)
All we have to do is to make sure we got the nearest number of this
form (stopping at the tenths) to our original number. 102.0304 is
between 102.0 and 102.1, and because 3 is less than 5, it is less than
halfway from one to the other. That makes it closer to 102.0, so
we don't have to round up.
So that's the answer: 102.0.
If you have any further questions, feel free to write back.
- Doctor Peterson, The Math Forum
http://mathforum.org/dr.math/